1. Field of the Invention
This invention generally relates to a method for a pulsed gamma-gamma density tool to simultaneously compensate for interactions due to the photoelectric effect and density variations caused by standoff, thereby enabling a more precise determination of bulk formation density. Also disclosed a compensated tool utilizing a betatron as a Bremsstrahlung source.
2. Background of the Invention
In the oil well industry, reservoir characterization is used to predict the location of oil-bearing and gas-bearing formations, estimate the producibility of these formations, and assess the quantity of hydrocarbon in the reservoir.
A basic parameter for reservoir characterization is bulk formation density. There are many methods to determine bulk formation density. One widely accepted method is gamma-gamma (γ-γ) density. Gamma rays are packets of electromagnetic radiation, also referred to as photons. A γ-γ density sonde has a radioactive source, such as Cs137, that emits gamma rays which are photons of energy at 662 keV, and two or more detectors located at various spacings from the radioactive source that count the number of photons that strike that detector as a function of time or energy. Typically, there is a short space (SS) detector located close to the radiation source and a long space (LS) detector further away from the radiation source. The SS detector generally has a shallower depth of investigation than the LS detector and is more sensitive to borehole fluid or mud cake between the sonde and the formation. The space between the sonde and the formation is called the standoff which normally filled up with borehole fluid, drilling fluid or mud cake. The LS detector has a deeper depth of investigation and is less sensitive to the borehole environment and more sensitive to the formation.
Both the radioactive source and the detectors are usually collimated and shielded to enhance the formation signals and to suppress borehole and tool housing signals. The geometry of the sonde mandates that a scoring photon (a photon striking a detector) must have interacted with at least one scattering electron before reaching a detector.
Two types of gamma ray interactions with earth formations dominate within the photon energy range of interest (from less than 100 keV to a few MeV). They are the photoelectric absorption (Pe) and Compton scattering. The probability of the type of interaction depends on the atomic number of the formation material and the energy of the gamma ray. For most earth formations, the photoelectric effect is dominant for gamma ray energies below about 100 keV. The photoelectric effect results from interaction of a gamma ray with an atom of the formation material. The incident gamma ray disappears and transfers its energy to a bound electron. The electron is ejected from the atom and replaced with another, less tightly bound, electron with the accompanying emission of a characteristic fluorescence x-ray with an energy dependent of the atomic number of the formation material.
The cross section for the photoelectric absorption, σPe varies strongly with the energy, falling off as nearly the cube of the gamma ray energy (Eγ). σPe is also highly dependent on the atomic number (Z) of the absorbing medium. For gamma rays with energies between 40 and 80 keV, the cross section per atom of atomic number Z is given by:σPe≈Z4.6/Eγ3.15  (Eq. 1)
Since Pe is very sensitive to the average atomic number of the formation medium, it can be used to obtain a direct measurement of lithology or rock type. This is because the principal rock matrices (such as sandstone, limestone and dolomite) have different atomic numbers and considerably different Pe absorption characteristics. Liquids filling pores in the formation medium have only a minor effect of Pe due to the low average atomic number of the liquids.
The presence of high Z elements along the photon transport path, such as is encountered in barite mud, has a significant impact on the detected signal strength and low energy photons are affected more than the high energy photons. Even photons at the highest energy, i.e. >500 keV, are not entirely immune to the photoelectric effect. A formation's photoelectric absorption influence on the measurement is characterized by its photoelectric factor (PEF). To obtain an accurate density measurement, it is necessary to know the formation's PEF. Although the Pe effect complicates density measurements, it does provide valuable information about the formation lithology.
Measuring a formation's PEF with a chemical radioactive source is not difficult. The source emits continuously, the average detector count rate is not very high and the density detector usually operates in a photon counting mode. In this mode, the detector records not just the total photon scores, but also the energies of individual scoring photons. By comparing the photon scores in different energy windows, it is possible to extract both PEF and density accurately.
At higher gamma ray energies, the dominant interaction is Compton scattering that involves interactions of gamma rays and individual electrons. A portion of the gamma ray energy is imparted to an electron and the remaining gamma ray is of reduced energy. A gamma ray of incident energy E0 interacts with an electron of the formation material, scatters at an angle θ, and leaves with an energy E′. The attenuation of gamma rays due to Compton scattering is a function of the bulk density (ρb) and the ratio of atomic number to atomic mass (Z/A). Z/A is approximately 0.5 for most formation materials of interest, so the bulk density may be calculated from:ΣCo=σCo(NAv/A)(σb)(Z)  (Eq. 2)where ΣCo is the macroscopic cross section, σCo is the Compton cross section and NAV is the average number of scoring photons at the detector.
Conventional γ-γ density tools have a significant drawback. They require a chemical radioactive source, that is difficult to dispose and hazardous if misused. There is a move to replace chemical radioactive sources with electronic sources. An electronic source produces photons by accelerating an electron beam to a suitable high energy and impinging the beam on a target. Two types of electronic sources are DC electrostatic accelerators and pulsed accelerators. A pulsed machine may employ a variety of means to achieve a high beam energy, for example, a betatron utilizes a changing magnetic field to accelerate electrons which are then impinged on a target to generate Bremsstrahlung photons with a continuous energy spectrum from 0 up to the electron beam energy. Typically, pulsed machines have a low duty cycle and the photons are produced in short bursts of a few microseconds or less. To achieve adequate statistics, the source must deliver on average, many scoring photons per burst. Since those photons arrive at the detector at nearly the same instant, they are indistinguishable from each other. For such machines, the detector operates in an energy deposition mode, the detectors only record the total energy deposited in one burst. Since the photon energy distribution information is not available, other mechanisms are required to separate PEF and density information embedded in the signals.
Extracting PEF and density information requires separating low energy photons from high energy photons. One simple approach is to use a low energy filter to cut off photons below a threshold energy. For example, U.S. Pat. No. 3,321,625 to Wahl discloses that the Pe effect is dominant when the photon energy is less than 50 keV and placing a silver or cadmium disc in front of the detectors will absorb photons with energies less than 50 keV thereby minimizing the Pe effect. However, the 50 keV is a statistical average and the detected signals are still affected by PEF albeit to a lesser degree. Using filters to completely remove photons below a certain threshold comes with a penalty, namely, many high energy photons that carry density information are also lost. Consequently, using filters to reduce the Pe effect does not meet the precision requirements of modern logging.
Another approach is to use a laminated detector. In one embodiment, the detector consists of two different scintillators, a low density “semi-transparent” scintillator facing the formation and a high density “absorbing” detector in the back. In theory, the low density scintillator absorbs mainly low energy photons and allow most high energy flux to transmit through to the rear detector. In practice, a significant amount of high energy flux is also absorbed by the low density scintillator rendering the technique less sensitive than desired.
There remains a need for a method and apparatus to compensate for PEF in a pulsed electronic accelerator, such as a betatron that maximizes the information that may be extracted from scoring photons and retains high sensitivity without sacrificing precision.